大兴安岭中段龙江盆地中侏罗统地层新研究

The Jurassic strata in the Great Xing'an Mountains of western Heilongjiang Province are not highly studied. In the past, the identification of the Middle Jurassic coal-bearing strata is mainly based on the correlation with the Wanbao Formation in western Jilin Province. In recent years, with the new round of the regional geological survey on coal, oil and gas stratigraphy in Greater Xing'an Mountains，I discovered a new Jurassic coal formation in the Longjiang Basin in the middle of the Great Xing'an Mountains (in the western Longjiang County, Heilongjiang Province), which consists of sandy conglomerates，volcanic-clastic and coal seams. The Neocalamites-Coniopteris-Raphaelia assemblage has been found in some plant fossils there, which indicates that the newfound formation is early and middle Jurassic. Besides, preliminary results on the sporopollen fossils also suggest the same period. Moreover, several volcanic rocks from that formation were sampled and then dated, which show about 165.2 Ma and 162.1 Ma. Based on the current preliminary results，the study on the Middle Jurassic strata in Longjiang basin of Great Xing'an Mountains is expected to make new progress. The project here aims to further clarify the times and characteristics of the strata in Longjiang basin through the comprehensive study of stratigraphy, paleontology and isotopic chronology, together with the comparison to the Middle Jurassic Wanbao Formation in the southern part of the Great Xing'an Mountains, in order to expand our knowledge on the connotation of the group and the distribution of paleogeography for the Wanbao Formation of Great Xing'an Mountains, and further to improve the comprehensive research degree of the Jurassic strata in the northern section of the Great Xing'an Mountains (western Heilongjiang).

我国黑龙江西部大兴安岭地区侏罗纪地层研究程度不高，以往中侏罗世含煤地层的确定主要借助于同吉林西部万宝组的对比。近年来随着新一轮大兴安岭地区含煤及油气地层的区域地质调查工作的开展，前不久申请者在大兴安岭地区中段的龙江盆地发现一套新的中侏罗世含煤地层，该地层由砂砾岩和火山碎屑岩夹煤层组成。从采得的植物大化石中已发现锥叶蕨-拟刺葵组合，显示早、中侏罗世时代；孢粉化石经初步鉴定，时代意见也倾向于早、中侏罗世；申请者又在本组火山岩采样送同位素测年，已获约165.2Ma和162.1Ma结果。上述初步研究表明，大兴安岭地区龙江盆地的中侏罗世地层研究有望取得新进展。本项目拟通过地层学、古生物学、同位素年代学等综合研究，进一步明确龙江盆地上述地层的时代及特征、以及与大兴安岭地区南段中侏罗世万宝组的对比，以扩大对大兴安岭地区万宝组的内涵及古地理分布等认识，提高我国大兴安岭地区中段侏罗纪地层的综合研究程度。

近年来突泉盆地中侏罗统中获得轻质原油，为松辽盆地外围侏罗纪盆地的新区、新层系油气找寻提供了新思路。与它毗邻的龙江盆地，也发育有类似的侏罗纪地层。本项目正是以龙江盆地新厘定的一套中侏罗世含煤地层为研究对象，首次综合性地详细研究了龙江盆地该套地层的植物化石（包括孢粉和木化石），通过与国内外中侏罗世含煤地层及其植物群的对比，确认龙江盆地上述植物群属于我国 Coniopteris - Phoenicopsis 植物群的早期组合，时代为中侏罗世；并获得了以166.2±1.3Ma为代表的一系列同位素测年数据，辅证了上述中侏罗世时代。本项目还系统地分析了龙江盆地中侏罗世时期沉积物源特点、古环境、古气候及构造背景等特征，确认了在龙江盆地对上述中侏罗世含煤地层使用万宝组的合理性。最具代表性成果是新发现了蚌壳蕨科的原位花粉，已建立2个新种；新发现了丰富的木化石，其中2个新种（Protocedroxylon zhalantunense Zhang, Tian et Wang和Protocedroxylon zhangii Zhang, Tian et Wang）均具有Abietineentüpfelung特征，该发现将具有这种特征的松柏类植物的时代提前至中侏罗世；孢粉化石首次识别出45属73种，建立了3个孢粉组合。上述新发现反映了龙江盆地万宝组植物群为一个北方暖温带温和、凉爽，并具有季节性气候变化的内陆植物群。本研究不仅为提高我国大兴安岭地区中段侏罗纪地层的综合研究程度、扩大找寻松辽盆地西缘煤及石油等矿产远景等将起到推动作用，而且为深入研究中国北方侏罗纪植物地理分区和古气候环境等提供了新资料。